The maximum intensity projection (MIP) method is commonly used as a three-dimensional postprocessing method to depict volumetric vascular data sets acquired with both computed tomography (CT) and magnetic resonance (MR) imaging. Both modalities tend to produce a large number of primary reconstructed sections, which has prompted a greater use of three-dimensional postprocessing. In addition, three-dimensional vascular anatomy is difficult to discern when only cross-sectional images are used. MIPs are capable of presenting angiogram-like views calculated from the primary data that make anatomic and pathologic features easier to identify. To produce MIPs, a viewing angle is chosen to define the projection plane. Parallel rays are then cast from the projection plane through the stack of reconstructed sections that make up the data volume, and the maximum intensity encountered along each ray is placed into the projection plane to construct the MIP. Vessels have higher contrast intensity values than those for soft tissue. Therefore, the MIP shows a projected two-dimensional view of the vessels as seen from the center of the projection plane. Since some information is lost in the conversion from three to two dimensions, MIPs can be computed from many viewing angles and shown in a cine loop to convey the three-dimensional anatomy of the vessels.
The contrast in MIPs decreases with increasing projected volume (MIP thickness) because the probability that the maximum value encountered in the background will match or exceed the vessel intensity increases with MIP thickness. Although MIPs exhibit an increased contrast-to-noise ratio compared with that of source images, predominantly as a result of decreased noise, the reduced contrast between vessels and background can result in artifacts. This effect can lead to the disappearance of vascular features that have intensities only as great as the intensity of the background. Therefore, small vessels, which have decreased intensity as a result of volume averaging, can become invisible. The edges of larger vessels, which are less intense than the vessel center because of volume averaging, may be obscured, which leads to apparent vessel narrowing. High-grade stenoses may be overestimated on MIPs and appear as segmental vessel occlusions.
Regions of interest (ROIs) can be defined around vessels to limit the MIP thickness, thereby improving contrast in the MIP. In CT angiography, this method also allows the exclusion of high-attenuating bone that otherwise could overlap and obscure the vessels. A rectangular oblique plane can be easily specified and thickened to enclose a cuboidal ROI that can be used to produce conventional rectangular-slab MIPs, which are also known as thin-slab MIPs. In regions of complex and tortuous anatomy and for certain viewing angles, however, cuboidal ROIs cannot maximally exclude bone (See e.g. Napel et al. (1992) in a paper entitled “CT angiography with spiral CT and maximum intensity projection” and published in Radiology 185:607–610) and may include excessive soft tissue. Usually, separate cuboidal ROIs have to be specified for each vessel of interest, which increases the number of MIP reconstructions per study. Alternatively, manual section-by-section editing can be performed to draw ROIs around structures to exclude or include them, but this is tedious, may not be reproducible, and may be susceptible to tracing errors (See e.g. Napel et al. (1992) in a paper entitled “CT angiography with spiral CT and maximum intensity projection” and published in Radiology 185:607–610). Accordingly, there is need for a new method that adaptively encloses vessels of interest while excluding bone and surrounding soft tissue.